linux-kernel - Re: [PATCH 1/1] arm64/sparsemem: reduce SECTION_SIZE

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Date:   Thu, 21 Jan 2021 15:54:10 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Christoph Lameter <cl@...ux.com>
Cc:     Sudarshan Rajagopalan <sudaraja@...eaurora.org>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mark Rutland <mark.rutland@....com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Price <steven.price@....com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH 1/1] arm64/sparsemem: reduce SECTION_SIZE_BITS

On Thu, Jan 21, 2021 at 10:08:17AM +0000, Christoph Lameter wrote:
> On Wed, 20 Jan 2021, Sudarshan Rajagopalan wrote:
> 
> > But there are other problems in reducing SECTION_SIZE_BIT. Reducing it by too
> > much would over populate /sys/devices/system/memory/ and also consume too many
> > page->flags bits in the !vmemmap case. Also section size needs to be multiple
> > of 128MB to have PMD based vmemmap mapping with CONFIG_ARM64_4K_PAGES.
> 
> There is also the issue of requiring more space in the TLB cache with
> smaller page sizes. Or does ARM resolve these into smaller TLB entries
> anyways (going on my x86 kwon how here)? Anyways if there are only a few
> TLB entries then the effect could
> be significant.

There is indeed more TLB pressure with smaller page sizes but this patch
doesn't change this.

-- 
Catalin